Desktop: side-panel "Delete agent" deletes only the agent instance, leaving the persona — the same button silently becomes "Start agent"

### Describe the bug

In the profile side panel that opens when you edit an agent, the destructive row labelled **Delete agent** does not delete the agent. It deletes only the *keyed instance* — which stops the process — and leaves the key-less *persona definition* in the store. The panel then re-renders with a **Start agent** button for the same name, and a second click of the same "Delete agent" row is required to actually remove it.

From the user's side this reads as: I confirmed a destructive "Delete agent" dialog, and all it did was shut the agent down. That is a dangerous shape for an action labelled and styled as destructive, and a confirm dialog that promises "Removes the local management record and saved agent key" / "Removes the agent from every channel it belongs to" is not describing what the first click does.

### Steps to reproduce

1. Open an agent's profile side panel (edit an agent) as its owner.
2. Scroll to the destructive **Delete agent** row; confirm in the "Delete this agent?" dialog.
3. The agent stops. Reopen its profile — the name is still there, and the primary action is now **Start agent**.
4. Click **Delete agent** again — this time it actually deletes.

### Expected behavior

One confirmed "Delete agent" removes the agent — instance *and* persona — or the dialog states plainly that only the running instance is being removed and the definition is kept.

### Mechanism (code refs, `origin/main` @ eed74bd)

The store is unified: key-less *definitions* (former personas) and keyed *instances* live in the same `managed-agents.json` (`managed_agents/storage.rs`, `load_managed_agents` filters to `!pubkey.is_empty()`, `load_agent_definitions` to `pubkey.is_empty()`).

- `delete_managed_agent` (`desktop/src-tauri/src/commands/agents.rs`) stops the process, then retains-out only the matching **keyed** record. `save_managed_agents` deliberately re-reads and preserves the definition half (`storage.rs:364`), so the persona survives the delete. Correct on its own terms — but it is the whole of what the first click does.
- `desktop/src/features/profile/ui/UserProfilePanel.tsx:718` — `handleDeleteProfileAgent = viewerIsOwner && managedAgent ? handleDeleteAgent : handleDeletePersona`. Same row, two different actions depending on whether an instance currently exists.
- `UserProfilePanel.tsx:333` — `canInstantiateAgent = isOwner && resolvedPersona !== undefined && managedAgent === undefined`. After the instance is gone this flips true, and `UserProfilePanelSections.tsx:415` renders `ProfilePersonaPrimaryActions` → **Start agent**.

So the button's identity changes underneath the user between click 1 and click 2, with no label or copy change to signal it.

### Secondary finding (same flow)

`desktop/src/features/profile/ui/UserProfilePanelDeletion.ts:89` hardcodes `skipRemoteDeleteConfirm: true`. The other delete path (`useManagedAgentActions.handleDelete`) passes no such flag, so it shows the "shutdown sent but the agent may still be running — deleting now orphans the remote deployment" confirms from `deleteManagedAgentWithRules` (`managedAgentControlActions.ts:171-201`). The side panel suppresses all of them. Same user-visible action, weaker orphan gate on the panel path.

### Related

- #5786 — relay-side residue after delete. Note its "Defect C: 12 of 48 rows have empty pubkey" is not corruption; those are the definition half of the unified store. The rest of that report stands.
- #3771 — provider-backed agent delete routed through persona deletion.

### Version / platform

macOS (Apple silicon). Code analysed against `block/buzz` `origin/main` @ eed74bd. Behaviour observed in Buzz Desktop by the reporter; the delete path was not instrumented, so the code refs above are the mechanism that matches the observed sequence.

Start with desktop/src/features/profile/ui/UserProfilePanel.tsx around handleDeleteProfileAgent and canInstantiateAgent, then trace UserProfilePanelDeletion.ts and delete_managed_agent in desktop/src-tauri/src/commands/agents.rs. Compare the instance and persona delete paths, including managedAgentControlActions.ts:171-201; done means one confirmed action has an explicit, consistent result and the remote-delete confirmation is not silently bypassed.

Written by the indexing model from the issue text.